Abstract
Direct solar radiation data observed since 1933 at the Japan Meteorological Agency (JMA) observing stations have been analyzed. The transformed turbidity factor ln(τ0--1), where τ0 is the Feussner-Dubois' turbidity coefficient, is introduced in order to normalize the frequency distribution. This allows for a better statistical analysis of atmospheric turbidity. The turbidity factor clearly shows the influence of three large volcanic eruptions: Agung (1963), El Chichon (1982), and Pinatubo (1991).
These volcanic effects appear in the time series of data as unusual seasonal variations. Direct solar radiation data at global stations should be continuously analysed in order to better understand the trend in stratospheric aerosols.
